The initial appearance of the journal bearing rock bit for drilling applications in 1963 revolutionized rock bit consumption and drilling costs. This type of bearing made it possible for the bearing structure to have a service life that exceeded the life of the cutting structure in many types of drill bits.
Rock bits with journal bearings consist of the following common component parts. Firstly, there are three head sections that are generally welded to form a unitized configuration. Each of these head sections has a cantilevered lower portion forming a mounting for the conical cutter, this mounting being the journal bearing pin. Associated areas of the bit construction consist of three independent lubrication systems, one for each bearing, and nozzles to direct fluid flow from the central bore of the rock bit body to the formation to remove drill cuttings.
Journal bearing pins of prior art rotary bits have five major characteristic features. These as viewed from the longitudinal centerline of the drill bit towards the outside diameter consist of a nose pin, an axial thrust surface, a locking system, a main journal bearing, and a seal surface. This general layout has historically proven to be the most satisfactory configuration.
Each of the five major areas serves a specific purpose and when operating in unison provides a technologically sound bearing structure. The nose pin adds marginally to the overall load capacity of the assembly but is primarily provided to add axial stability to reduce cone wobble during drilling. In certain cases, the axial thrust capacity of the bearing is increased by adding a thrust surface to the end of the nose pin. The main axial thrust surface acts as a transition surface between the nose pin and the locking system. This thrust surface is required to resist any outward forces of the cone on the bearing assembly. Extremely high pressures are encountered here and special hardfacing or metallurgy of the bearing assembly is required in this area.
The locking system to retain the cone on the head section generally consists of an annular row of ball bearings. The ball race in the head section is positioned to co-act with a corresponding race in the cone such that when the balls are installed the cone is prevented from moving axially inward on the bearing pin. The main axial thrust face prevents outward movement.
The main journal bearing surface is the predominant area of the bearing structure. This area resists the total downward bit load and is completely dependent on the lubrication system provided in the drill bit. Numerous ways have been used to resist the forces that this bearing surface is required to operate under. Early bearings had a recess machined in the bearing pin and this recess was filled with a tungsten carbide hardfacing. This operated against a carburized and hardened journal surface in the cone. A solid lubricant of copper or silver was inlaid in the cone journal to resist seizure or galling during working operation. Later bearings use metallurgical techniques such as boronizing or nitriding to generate hard bearing surfaces in this area.
The seal area is generally an extension of the main journal bearing surface and is characterized as a highly polished area such that a low friction surface is provided for the seal to run against.